The watermark in the photo has the function of preventing sneak shots. If there is information such as phone number or source in the watermark, it can also play a propaganda role.
It is conducive to protecting one's original rights and protecting one's real interests from damage. To protect your own pictures, you can apply for patent protection for appearance, or you can infringe upon the rights of others. As long as you don't use other people's copyrights and don't copy them, there is no infringement. There is no infringement if you don't use other people's pictures for commercial activities.
Copyright is essentially an intangible property. As long as it is not their own property or the property of others, it is owned by the owner, so it cannot be used casually and does not necessarily need to be marked. Therefore, as long as you quote a work created by others rather than yourself, you should think that the work is protected by copyright and should be authorized unless you can confirm that the work has passed the copyright protection period.